Apparatus for, and a method of, cutting a blank

Abstract

When a blank is cut from metal sheet which suffers from crystallographic anisotropy, there is a tendency for ears to be formed during subsequent forming operations, such as drawing or pressing. In order to compensate for this tendency, there may be used a metal blank which is not completely round, but has lobes at positions to cancel at least some of the valleys between the ears. There is described a punch and die which may be used to form such a lobed blank. The punch has four circumferentially extending lobe-forming sections. Each lobe-forming section is constructed by forming a stepped recess in the cutting edge of the punch. In each lobe-forming section, the depth of the recess varies from a maximum at the middle of the section to zero at the ends. The die is conventional.

What is claimed is: 
     
       1. An apparatus for cutting a blank from metal strip or sheet, said apparatus comprising a punch having a substantially circular cutting edge and a die having a substantially circular cutting edge which is arranged to cooperate with the cutting edge of the punch, in which a plurality of circumferentially extending spaced-apart lobe-forming sections are provided in the cutting edge of said punch, each lobe-forming section being constructed by forming a radial recess in the cutting edge of said punch, the recess defining an arc of less than 180 degrees, said punch and die being arranged to cooperate to produce a blank having lobes at positions corresponding to the positions of said lobe-forming sections. 
     
     
       2. Apparatus as claimed in claim 1 in which, in each lobe-forming section, the area in cross-section which is absent from said punch by virtue of the recess increases progressively from the ends of the lobe-forming section towards the middle thereof. 
     
     
       3. An apparatus as claimed in claim 2 in which, in each lobe-forming section, the recess has a stepped shape in cross-section. 
     
     
       4. An apparatus as claimed in claim 3 in which, in each lobe-forming section, the width of the recess is substantially constant while the depth of the recess increases progressively from the ends of the lobe-forming section towards the middle thereof. 
     
     
       5. An apparatus as claimed in claim 3 in which, in each lobe-forming section, the depth of the recess is constant while the width of the recess increases progressively from the ends of the lobe-forming section towards the middle thereof. 
     
     
       6. An apparatus as claimed in any one of claims 1 to 3 in which the lobe-forming sections are spaced from each other by parts of the cutting edge of said punch in which a recess is not formed. 
     
     
       7. A method of cutting a blank from metal strip or sheet comprising the steps of taking a punch having a substantially circular cutting edge and a die having a substantially circular cutting edge which is arranged to cooperate with the cutting edge of the punch, in which a plurality of circumferentially extending spaced-apart lobe-forming sections are provided in the cutting edge of said punch, each lobe-forming section being constructed by forming radial recess in the cutting edge of said punch, the recess defining an arc of less than 180 degrees, placing the metal sheet between the punch and the die, orienting the punch relative to the metal sheet so that the lobes are formed in the blank at positions which compensate for valleys formed during subsequent metal working operations, and causing the punch and die to cooperate so as to cut a blank from the metal strip or sheet, said blank having lobes at positions corresponding to the positions of said lobe-forming sections. 
     
     
       8. A method as claimed in claim 7 in which, in each lobe-forming section, the area in cross-section which is absent from said punch by virtue of the recess increases progressively from the edge of the lobe-forming section towards the middle thereof. 
     
     
       9. A method as claimed in claim 7 in which, the lobe-forming sections are spaced from each other by parts of the cutting edge of said punch in which a recess is not formed. 
     
     
       10. A method as claimed in claim 7 in which the blank is subsequently subjected to a drawing operation. 
     
     
       11. A method as claimed in claim 7 in which the blank is subsequently formed from a can body by a drawing operation and one or more redrawing operations. 
     
     
       12. A method as claimed in claim 7 in which the blank is subsequently formed into a can body by a drawing operation, a redrawing operation, and one or more wall ironing operations. 
     
     
       13. A method as claimed in claim 7 in which the blank is subsequently formed into a can cover by a drawing operation and one or more redrawings operations.